This invention relates generally to orthopedic fixation devices and bone plating systems for fracture fixation, and particularly to systems and methods for using bone plates that provide polyaxial fixation of fasteners.
Bone fractures are often repaired by securing a bone plate across the fracture. Depending upon which bone is to be treated, the bone plate may be straight or curved to match the contour of the bone for which it is designed. Bone plates may also be provided in many shapes and sizes. In cases where a bone is severely comminuted or if bone segments are missing, the use of bone plate and screw systems promotes healing of the fracture by providing a rigid fixation or support structure between the bone and the plate.
Bone plates may be secured to the bone in a number of ways. An existing solution is a plate and screw system where the screws are locked in the plate. A bone screw is threaded through an opening in the plate and into the bone. The screw is then secured to the bone plate via threads in the screw head that cooperate with threaded openings in the bone plate. This secures the plate with respect to the bone and provides rigid fixation because the relationship between the plate and screw(s) is fixed. Because the head of the locking screw interdigitates with threads in the plate, the plate and screws(s) form a stable system or construct, and the stability of the fracture can be dependent on or aided by the stiffness of the construct. Locking a screw into the plate can achieve angular and axial stability and eliminate the possibility for the screw to toggle, slide, or be dislodged, reducing the risk of postoperative loss of reduction.
However, although locking screws may reduce the incidence of loosening, they provide only one fixed angle relationship between the plate and the screw(s). The insertion angle is limited to a single direction because the threads of the head cooperate or mate with the threads of the hole in one direction only. The longitudinal axis of the screw lines up with the central axis of the hole, and no angular variation is allowed. In short, locking screws are unidirectional, limiting their use in some instances.
For example, when treating a severe fracture, fragments may be shattered and in irregular positions. Although a surgeon may wish to obtain the benefits of a locking screw and bone plate used together, the angle at which the locking screw extends from the plate at a certain opening may not be the angle that would allow the surgeon to “grab” (or seize, fix, or otherwise secure) the desired, random bone fragment. In this case, the surgeon may need to secure the plate to the bone somewhere else, or use a non-locking screw. Although non-locking screws do not lock into the plate, they can be inserted at various angles.
Specifically, non-locking screws are secured into bone in the same way that locking screws are, but they are not secured to the plate. Their heads are typically rounded where they contact the bone plate. Thus, one advantage of non-locking screws is that they can be inserted at various angles because they are not limited by the thread-to-thread contact of locking screws with the bone plate. However, if the surgeon desires the rigid stable construct of a locking screw and plate, the use of a non-locking screw to obtain the desired angular orientation is not necessarily optimal.
There have been bone plating systems developed that provide the surgeon with the option of choosing a non-locking or a locking screw. In some embodiments, these systems provide plates with some threaded holes (that may receive with either locking screws or non-locking screws) and some non-threaded holes (for non-locking screws). There are also systems that provide partially threaded slots to allow either non-locking or locking screws to be used together. Such combination slots provide surgeons with the intraoperative choice about whether to use the plate with locking screws, non-locking screws, or with a combination of both. These combination slots typically have a partially threaded opening that can receive either a compression screw or a locking screw. However, because these combination slots are only partially threaded, the locking screw(s) may not be able to maintain the fixed angular relationship between the screw(s) and plate under physiological loads. Specifically, the locking screws within the plate are only partially captured and thus only partially surrounded by threads. Under high stress and loading conditions, the slot may distort and allow the fixed angular relationship between the locking screw and plate to change. This can result in loss of fixation or loss of established intraoperative plate orientation. Moreover, the locking screw can still only be inserted at a single angle—the predetermined angle defined by the manufacturer.
Additionally, current bone plate and screw systems still limit a surgeon's ability to both (a) lock a fastener with respect to the bone plate, but still (b) allow the fastener to extend from the bone plate at various angles. Locking screws lock into the plate, but only in a single angular configuration, and non-locking screws allow various angle configurations, but they do not provide a stable construct with the plate. Accordingly, none of these options allow a surgeon to capture bone fragments that do not fall in line with the axis of the opening provided on the plate in a rigid fashion. An example of this problem is shown in FIG. 21. Thus, currently available options can still lead to malalignment and poor clinical results.
There have, however, been some attempts to provide polyaxial locking systems. For example, one effort includes providing holes that accept fixed angle locking pegs and multidirectional locking pegs, with a threaded cap inserted over the multidirectional peg to hold it into place. Such a system can be cumbersome to use because although the multidirectional peg can be inserted at any angle, the surgeon then needs to thread a small cap onto the top of the peg head and into the plate, requiring an extra step, extra time, and extra instrumentation. Such systems also fail to allow the use of non-locking members in conjunction with the locking and multidirectional pegs.
Other systems that have attempted to offer polyaxial fixation include providing a bone plate with inserts at the hole peripheries made out of a deformable material, with the remaining part of the plate made of titanium. The plate is manufactured and the inserts are then pushed into the hole peripheries and engaged in place by deformation and pressure. When screws are inserted, the inserts deform and are compressed between the edges of the holes of the plate, which holds the screws and inserts in place. Challenges with such systems are that they cannot be used with non-locking screws, the inserts do not have the strength to receive and hold a regular locking screw, (i.e., they do not provide the surgeon with options), and plates with deformable inserts are more expensive to manufacture than regular bone plates. Other attempts have failed to provide adequate locking mechanisms.
Another attempt at polyaxial fixation includes a plate with holes that have an internal jacket with recesses that extend away from the axis of the hole or into the internal jacket surface. This attempt is described in International Application WO 2005/018472, titled Bone Plate. The internal jacket surface of the plate described in that application is threaded or has ribs or protuberances. A bone screw is intended to be pulled into the hole of the plate by the internal jacket surface. If the bone screw head is threaded, when the screw in inclined, the threaded head is intended to “jump over” the pitches of the threads in the hole of the plate interrupted by the recesses, without “cutting through” them. The goal of the invention is to provide a bone plate that can have bone screws introduced at an angle that is different from the specified axis of the hole and secured into position.
It would be beneficial to provide plates and methods that combine polyaxial locking fixation with a thinner profile and enhanced bone contouring. Such plates could be useful in fixation of partial articular and/or non full body weight bearing fractures, where a more flexible plate that is more closely shaped in accordance with bone structure and that features polyaxial locking openings could provide additional options for the surgeon. Such features could, if desired, but not necessarily, provide plates that allow compression of a mid portion against bone to create a buttress effect while other portions of the plate are locked to the bone using polyaxial fixation. Such plates could also be used without buttressing effects, but in a more conventional reinforcement mode. Other features could be combined with any or all of these features.